1. Field of the Invention
The present invention relates to an object detection apparatus and method for detecting a movable object existing in a region around a vehicle from images obtained through a camera mounted on the vehicle, and an object detection program for making a computer execute processing in the object detection apparatus.
2. Description of the Related Art
In recent years, techniques to detect an object approaching a vehicle by monitoring the surroundings of the vehicle and to perform operations such as presentation of information to the driver and control of travel of the vehicle on the basis of the detection results have been proposed. As a method of detecting an object, techniques are known in which images of a region surrounding a vehicle are taken with a camera mounted on the vehicle and the taken images are processed to detect a movable object. As one of such techniques, a technique using an optical flow computed from a time series of images obtained through a single camera can be mentioned. An optical flow is a vector indicating an apparent motion on an image detected from a plurality of images taken at different times. In the case of taking images with a camera mounted on a vehicle, the camera itself moves and, therefore, an apparent motion is produced on the images by the movement of the camera. For this reason, a technique in which the amount of movement of a camera itself is estimated in detection of a movable object by using an optical flow, and the movable object is suitably detected by eliminating the influence of the movement of the camera has been proposed (see, for example, Japanese Patent Laid-Open No. 2004-56763 (hereinafter referred to as Patent Document 1)).
A monitoring apparatus for use in a vehicle disclosed in Patent Document 1 has an optical flow detection unit which obtains an optical flow from images taken with a camera, a background flow estimation unit which obtains a background flow, i.e., a camera image flow (a flow produced by the movement of the camera itself) and an approaching object detection unit which detects movement of an object existing in a region around the vehicle by comparing the optical flow and the background flow. The background flow estimation unit obtains the background flow on the basis of the movement of the vehicle estimated by a vehicle movement estimation unit and a spatial model estimated by a spatial model estimation unit (a model for the space from which images are taken by the camera).
In the apparatus disclosed in Patent Document 1, however, the vehicle movement estimation unit estimates the movement of the vehicle using the monitoring apparatus on the basis of the wheel speed, steering angle and other factors, and obtains the background flow from the movement of the vehicle. In this case, the accuracy of estimation of the background flow is reduced during high-speed travel or travel on a curve in a road due to the accuracies of sensors for detecting the vehicle speed, steering angle and other factors and the influence of deviation between the camera rotation center and the vehicle rotation center. Also, in the apparatus disclosed in Patent Document 1, there is a need to measure the distance to objects existing on the road in the forward direction from the vehicle, obtain road information from a GPS or the like and estimate a spatial model in the spatial model estimation unit using preliminary knowledge about spaces and models for roads, walls and so on in order to obtain the background flow. Complicated processing is therefore required. There is also a possibility of failure to accurately estimate the background flow. In the apparatus disclosed in Patent Document 1, the movement of an object is detected by comparing the optical flow and the background flow and there is, therefore, a possibility of occurrence of a large error resulting from erroneous estimation of the background flow and, hence, failure to suitably detect a movable object.